As is well known in the art, various elements of a grinding mill typically are subjected to wear in characteristic patterns, in which certain surfaces of certain elements are subjected to greater wear than other surfaces.
As can be seen in FIGS. 1A-1D, a conventional discharge wall assembly 20 in a typical grinding mill 21 (FIG. 1D) includes a number of vanes or pulp lifters 22 (FIGS. 1A-1C) that extend inwardly (i.e., toward a central hole 24) from a shell wall or outer perimeter wall 26 of a mill shell 23. The vanes or pulp lifters 22 are at least partially mounted on a discharge end wall 27. The vanes are intended to direct pulp including ore particles and water to the central hole 24, through which the pulp exits the grinding mill. In the example illustrated in FIGS. 1A-1C, the vanes 22 include shorter and longer vanes. As is well known in the art, various arrangements of longer and shorter vanes, and possible additional vanes of intermediate length (not shown in FIGS. 1A-1C), may be used. The optimum design depends on a number of parameters, e.g., the hardness of the ore, and the unit cost of energy inputs, as is also known.
As is well known in the art, the vanes or pulp lifters 22, the outer perimeter wall 26, and the discharge end wall 27, at least partially define the pulp chambers 28 therebetween. Typically, discharge grates “DG” (FIG. 1D) are located on the pulp chambers 28 to screen the flow of slurry or pulp into the pulp chambers, i.e., to limit the solid particles in the slurry or pulp entering the pulp chambers to particles sized smaller than the apertures in the grates.
It will be understood that the majority of the solid particles in the pulp (i.e., primarily ore that has been ground), which exit the pulp chambers via the central hole 24, are omitted from FIGS. 1A-1C for clarity of illustration. As is well known in the art, the slurry or pulp is a heterogeneous mixture of solid particles and water. Some finer particles may be suspended in the water. The ore and the ore particles typically include some waste material.
As is well known in the art, the mill shell 23 of the grinding mill 21 defines a mill shell chamber 25 upstream from the pulp chambers, and the mill shell 23 is rotatable about an axis of rotation “AX” (FIG. 1D). When the grinding mill is operating, a charge “CH” is located in the mill shell chamber 25. The charge (i.e., ore, water, and grinding media, if grinding media are used) may fill the mill shell chamber up to a level indicated by a line “A” in FIGS. 1A-1D. The direction of rotation of the mill shell 23 is indicated by arrow “B” in FIGS. 1A-1C. Typically, the ore is added into the grinding mill at an input end (as schematically represented by arrow “IN” in FIG. 1D), and water is also added into the grinding mill. The charge is rotated as the mill shell of the grinding mill rotates, subjecting the ore to comminution and resulting in finely-ground ore particles that are included in a slurry that is passed to an output, or discharge, end of the grinding mill. The movement of the ore particles and water through the discharge grates “DG” and into the pulp chambers is schematically represented by arrows “OP” in FIG. 1D. As the mill shell rotates, the pulp chambers are also rotated.
As each of the pulp chambers is immersed in the charge in turn, the slurry flows into each pulp chamber successively. As can be seen in FIGS. 1A-1C, depending on the amount of the charge in the mill shell chamber, a pulp chamber may be immersed (in whole or in part) as it is rotated from about the three o'clock position to about the nine o'clock position. When the pulp chambers are rotated to be above the charge, the pulp in them partially exits (i.e., is partially discharged). As a pulp chamber is moved from about the nine o'clock position to about the three o'clock position (i.e., when it is located above the line designated “A”), the pulp in that pulp chamber is directed by gravity toward the central hole by the vanes that partially define that pulp chamber (i.e., one such vane being located on each side of the pulp chamber).
The vanes or pulp lifters also support the pulp that is positioned on them respectively, and direct the pulp toward the central hole, when the vanes are rotated through positions above the charge. The movement of the pulp from the pulp chambers and into the central hole 24 is schematically represented by arrow “EX” in FIG. 1D.
As is also well known in the art, due to the concentration of wear on certain surfaces of certain elements, the elements may need to be replaced, even though other parts of the elements have been subjected to relatively little wear. The result is that significant costs may be incurred due to excessive wear that is concentrated in a relatively small area of a surface of an element. First, costs are incurred in connection with purchasing a new element, e.g., all or part of a vane or pulp lifter. Second, costs are also incurred in connection with the replaced element, e.g., although the replaced element may be worn in only a small portion thereof, it is prematurely replaced, as other portions of the elements may not be worn out. Third, significant costs are incurred due to the downtime required to replace an element that is prematurely worn.
For example, the characteristic movements of certain of the ore particles in the pulp in the pulp chambers are illustrated in FIGS. 1A-1C. It is believed that at least some of the wear to which the elements forming the pulp chambers is subjected is due to the movement of carryover pulp.
It will be understood that the top surface of the charge (identified as “A” in FIGS. 1A-1D) typically varies significantly, depending on a number of parameters, and the level illustrated in FIGS. 1A-1D is exemplary only. (As will be described, embodiments of the invention are illustrated in the balance of the attached drawings.) In addition, those skilled in the art would appreciate that the direction of rotation may be clockwise or counter-clockwise, depending on how the mill is manufactured and installed.
“Carryover” of pulp in grinding mills (i.e., the incomplete discharge of pulp in pulp chambers within one revolution of a mill shell) is a serious problem. The extent of carryover may be as high as 50% or more, depending on the circumstances. Carryover imposes many costs on the operator. In particular, it appears that some of the wear to which the elements mounted on the discharge end wall are subjected is due to carryover.
As is well known in the art, ideally, all the pulp in a particular pulp chamber should empty out of that pulp chamber 28 in the time that such pulp chamber 28 is moved from approximately the nine o'clock position to approximately the three o'clock position. That is, ideally, the pulp chamber should be fully emptied before it is next re-immersed in the charge. However, in practice, it often happens that a significant portion of the pulp does not exit the pulp chamber by the time that the pulp chamber has reached the three o'clock position. The pulp remaining in the pulp chamber, at a point when it ideally all should have been discharged via the central hole, is typically referred to as “carryover”.
The movement of the pulp that is carried over is schematically illustrated in FIGS. 1A-1C. It will be understood that the illustrations in FIGS. 1A-1C are based on computer-generated graphic simulations of the movement of the pulp in the pulp chambers as the mill shell rotates.
The reasons for carryover are well-known in the art. The relatively high mill shell rotation speed, e.g., about 10 rpm, is an important factor. This relatively fast rotation speed means that the discharge wall 27 completes one rotation every six seconds. Accordingly, the pulp in a particular pulp chamber has only approximately three seconds, at most, to exit the pulp chamber 28, i.e., to be moved to the central hole 24 and to exit therethrough. In addition, due to the rotation of the mill shell, the pulp in each pulp chamber is urged outwardly by centrifugal force, i.e., away from the central hole 24, effectively slowing the exit of the pulp from the pulp chamber as the pulp chamber moves from approximately the nine o'clock position to approximately the three o'clock position.
It has been determined that the movement of the pulp that is carried over, inside the pulp chamber, is distinctive to the specific grinding mill, and generally consistent. Because of this, the elements of the discharge wall assembly 20 in a particular mill are generally subjected to wear in substantially consistent patterns over time. However, the wear is not necessarily uniform over different pulp chambers in a particular mill, for reasons that are unclear. For example, one pulp chamber may be subject to excessive wear in the outer region thereof (i.e., proximal to the outer perimeter), and the pulp chambers adjacent thereto may not be subjected to excessive wear, or may be subjected to excessive wear in other areas thereof.
For example, in FIG. 1A, pulp chambers identified for convenience by reference numerals 28A-28E are shown with ore particles 30 of the pulp therein. (It will be understood that only a portion of the ore particles that are in the pulp chambers are illustrated in FIGS. 1A-1C, for clarity of illustration. Also, the water in the pulp is omitted from FIGS. 1A-1C, for clarity of illustration.) As can be seen in FIG. 1A, as an example, pulp chamber 28A is partially defined between a pair of the vanes or pulp lifters identified for convenience by reference numerals 122 and 122A, which are the trailing and leading vanes respectively, relative to the direction of rotation. When the pulp chamber 28A is in the one o'clock position, the solid particles 30 start to fall from a leading edge 132 of the vane 122 (FIG. 1A).
In pulp chamber 28B, partially defined between a pair of the vanes identified in FIG. 1A for convenience as 122A and 122B, the movement of the solid particles 30 toward a trailing side 134B of the leading vane 122B is more pronounced, because the pulp chamber 28B as illustrated is further along the clockwise rotation than the pulp chamber 28A. (It will be understood that of the pair of the vanes that define the pulp chamber 28B, the vane 122A is the trailing vane, and the vane 122B is the leading vane.)
In FIGS. 1A and 1B, pulp chambers 28C, 28D, and 28E show the solid particles 30 progressively moved further onto the trailing edge of the leading vane in each pulp chamber respectively, due to the changing positions of the pulp chambers as the mill shell rotates and the effects of gravity on the solid particles 30. In particular, in FIGS. 1A and 1B, it can be seen that, in the pulp chambers 28D, 28E (located at the three o'clock position, or almost at such position) the particles 30 that will be carryover are positioned in a middle area 35 of the trailing edge 134 of the leading pulp lifter, and they are spaced apart from the shell wall 26 by a distance 36 (FIG. 1B).
As can be seen in FIG. 1C, the ore particles 30 move downwardly, to pile on the shell wall 26, when the pulp chambers are at or close to the six o'clock position. Those skilled in the art would also appreciate that the slurry that flows into the pulp chambers, to fill them when the pulp chambers are positioned below the surface of the charge is also omitted from FIGS. 1A-1C. It will be understood that, although omitted, the pulp (the ore particles and water) quickly fill the immersed pulp chambers.
It can be seen in FIGS. 1A-1C that, although the solid particles 30 in a particular pulp chamber have been moved part of the distance toward the central hole when the pulp chambers are at approximately the three o'clock position or prior thereto, the particles 30 that are illustrated as becoming carryover do not reach the central hole.
The particles 30 that are destined to become carryover in the illustrated example are, at one point while the mill shell rotates, generally located in the middle area 35 of the pulp lifter, i.e., they are temporarily located a relatively short distance from the central hole. From FIGS. 1A and 1B, it can be seen that the particles 30 have moved from the shell wall 26 to the middle area 35 as the pulp chamber 28 in which the particles 30 are located has moved from approximately the nine o'clock position to approximately the three o'clock position. However, because the particles 30 that are illustrated have not reached the central hole 24 when the pulp chamber they are in is at the three o'clock position, they are returned to engage the outer perimeter wall 26 as the pulp chamber in which they are located moves further (clockwise) from approximately the three o'clock position. For these particles 30, the gains achieved during this rotation (i.e., the distances moved toward the central hole) are lost when the pulp chamber moves past the three o'clock position.
It will also be appreciated that the carried-over solid particles 30 move to the outer wall 26 when the pulp chamber(s) in which they are located is next re-immersed in the charge, as illustrated in FIG. 1C. The carried-over particles 30 will only exit the mill (i.e., via the central hole 24) in the next rotation if such solid particles reach the central hole during such rotation. Accordingly, it can be seen that some of the pulp that is carried over to the subsequent rotation may be carried over for several rotations.
In FIGS. 1A-1C, it can also be seen that the carryover of the ore particles 30 results in increased wear on certain portions of the pulp lifters 22, and also on the shell wall 26. For instance, in FIG. 1A, the solid particles 30 of the carryover fall from the leading side 132 of the pulp lifter 122, and such particles 30 engage the trailing side 134 of the adjacent pulp lifter 122A. In this way, a portion “C” of the trailing edge of each leading pulp lifter is subjected to wear due to the solid particles 30 that are carried over, by the sliding movement of the ore particles on the portion “C”. The portion “C” is generally spaced apart from the shell wall 26, i.e., the portion “C” is generally at the intermediate part 35 of the pulp lifter.
It can also be seen in FIG. 1A that the trailing side 134 of the pulp lifter 122 is subjected to impact (or dynamic) loading of the ore particles 30 onto the trailing side 134 of the pulp lifter, at a location on the trailing side 134 identified as “I” in FIG. 1A.
As can be seen in FIG. 1C, the solid particles 30 that are carried over tend to accumulate in the pulp chamber 28 on the mill shell wall 26, when the pulp chamber 28 is at or near the six o'clock position. (As noted above, other ore particles moved into the pulp chambers when they are immersed in the charge are omitted from FIGS. 1A-1C for clarity of illustration.) The portions “D1”, “D2” of the pulp lifters partially defining the pulp chamber that are proximal to the mill shell wall 26 may also be subjected to wear due to carryover, as are the portions of the mill shell “E” (FIG. 1C) that partially defines the pulp chamber 28.
In FIG. 1A, certain ore particles that are not destined to be included in carryover are also illustrated, identified by the reference numeral 31. The ore particles 31 move downwardly toward the central hole 24, as schematically represented by arrows “J” in FIG. 1A. However, due to the lengths of adjacent pulp lifters, those pulp lifters are subjected to impact loading of the ore particles onto the trailing side 134 of the pulp lifters 22, at locations on the trailing sides 134 identified as “K” in FIG. 1A. Accordingly, as illustrated, the pulp lifters are subjected to excess wear proximal to their respective inner ends, at “K”.